Total organic carbon (TOC) is a valuable measurement in environmental engineering and Earth sciences. The decay of the particulate fraction of TOC is the underlying process behind BOD (Biochemical Oxygen Demand) the most commonly regulated parameter for processed wastewater and industrial effluent. TOC is a fundamental parameter of water quality, summing both anthropogenic and natural pools of organic carbon compounds. TOC is also used extensively in the aquatic sciences as an indicator of biological activity. Total Organic Carbon, as used herein, is the concentration of all organic carbon compounds in a given sample, both dissolved and particulate. Organic carbon includes everything from decaying vegetation and live phytoplankton to man-made substances such as pharmaceuticals, pesticides, and Disinfection Byproducts (DBPs). Depending on the application, TOC concentrations of interest range from parts-per-billion to thousands of parts-per-million.
Because TOC is such a fundamental component of water quality assessment, there is a long and varied history of its measurement. The first widely-used automated determinations of TOC were made in the early 1970s with analyzers that used chemical oxidation to convert TOC compounds to carbon dioxide followed by measurement of the gas with Non-Dispersive Infrared (NDIR). Chemical oxidation was the standard method for breaking down TOC until a series of studies in the late 1980s found that the traditional chemical oxidation method did not always completely oxidize all TOC to CO2. These findings led to a modified oxidation method involving the use of high temperature catalytic oxidation (HTCO). See “A High Temperature Catalytic Oxidation Method for the Determination of Non-Volatile Dissolved Organic Carbon in Seawater by Direct Injection of a Liquid Sample”, Sugimura Y. and Suzuki Y., Mar. Chem., 24, 105-131 (1988). While the oxidation step in chemical oxidation and HTCO are different both techniques use NDIR.
Oxidative analyzers with NDIR detection remain the standard technology for measurement of TOC. While effective, this approach suffers from several shortcomings for long-term monitoring applications: (1) TOC analyzers require skilled operators and need a high level of maintenance and calibration. (2) These analyzers make extensive use of chemicals that are environmentally unfriendly and expensive. (3) Measuring TOC by analyzer is time-consuming and laboratories typically require several days to produce results. (4) The hardware used in this approach is cumbersome involving small-bore tubing and compressed gases. These requirements have made it difficult to modify the approach for continuous monitoring.
Direct optical measurement has the potential to alleviate or eliminate these problems and there have been multiple such approaches applied to monitoring TOC. Direct optical measurements normally focus on the ultraviolet part of the spectrum and can be grouped into fluorescence and absorption techniques.
Fluorescence is attractive because of its high sensitivity and relatively good S/N (signal to noise ratio) and there have been many studies relating the TOC of natural waters to the blue emission developed from UV excitation. See “Characterization of Marine and Terrestrial DOM in Seawater Using Excitation-Emission Matrix Spectroscopy”, Coble, P. G., Mar, Chem., 51, 325-346, (1996). The term commonly used for TOC measured in this way is Fluorescent Dissolved Organic matter (FDOM). The FDOM parameter mimics TOC quite well in natural waters ranging between 0.5 and 5 mg/L, but the FDOM/TOC ratio varies with the type of organic matter. Marine organic carbon and terrestrial organic carbon, for example, have different fluorescence efficiencies and these differences complicate the FDOM-TOC relationship where these types mix—as in estuaries and the coastal ocean. FDOM is also broken down by sunlight (see, “Comparison of Photochemical Behavior of Various Humic Substances in Water: III. Spectroscopic Properties of Humic Substances”, Zepp, R. G. and Schlotzhauer, P. F., Chemosphere 10, 479-486 (1981)) and this process of photo-oxidation further complicates the relationship with more conservative TOC. Moreover, anthropogenic organic compounds like pesticides often have complex cyclic structures that are generally more fluorescent than the straight-chain molecules that make up the bulk of natural organic matter. In summary, fluorescence works well as an analog for TOC in some cases but cannot be standardized for general use.
Organic compounds also absorb strongly in the UV and this absorption has also been exploited as an analog measurement for TOC. Differential absorbance spectroscopy, a measurement of water monitoring as a function of wavelength has been described previously. See “Development of Differential UV Spectroscopy for DBP Monitoring”, Korshin, G. V., et al., AWWA Foundation, (2002). Monitoring TOC using UV absorption has focused on wavelengths between 200 and 400 nanometers where strong absorption bands of organic compounds occur. Historically this range also represented that part of the UV spectrum accessible by readily available optical components. However, absorptivity of water in this region of electromagnetic spectrum is a potential problem. Consider the plot of water absorptivity versus electromagnetic wavelength shown in the graph of FIG. 1 (derived from http://www.lsbu.ac.uk/water/vibrat.html#uv). The region of wavelengths between 200 nm and 400 nm is region of particular interest because it offers a window of minimum absorptivity of water. This means that UV light in this range penetrates relatively deeply into the water and enables a greater volume of water to be probed for materials. The net result is more scattering, and absorption measurements in this region are particularly sensitive to interference by particles in the water. It is well known that cyclic organic compounds absorb strongly in this region and that this specific absorption is centered at 254 nm. This property was developed into a water quality index called Specific UV Absorption or SUVA. SUVA is a measure of the relative aromaticity of the organic molecules in the organic carbon pool. Complex man-made hydrocarbons such as those present in pesticides and pharmaceuticals absorb more UV light than the simple long-chain molecules of natural origin. SUVA is calculated by normalizing UV absorption at a wavelength of 254 nm by the mass of organic carbon (see, EPA Document #: EPA/600/R-05/055, Method 415.3, “Determination of Total Organic Carbon and Specific UV Absorbance at 254 nm in Source Water and Drinking Water”, Rev. 1.1, February 2005). In this way SUVA provides a qualitative measurement of the carbon pool that is particularly sensitive to artificial contaminants, where higher SUVA indicates water of lower quality. However, a greater volume of water being probed results in more scattering from that volume. Further, complex ring compounds tend to be strong absorbers and highly fluorescent thereby negating any simple relationship between TOC and fluorescence or UV absorption between 200 and 400 nm.
Particular issues exist with current instruments covering the UV-visual portion of the spectra. Because of the widely varying absorption coefficient of water, instruments employing a fixed path length measurement optical sample cell limits either absorption or the fluorescence signal.
Further, several relationships have been established in the art. The Beer-Lambert Law relates to absorbance readings. This is most often used in a quantitative way to determine concentrations of an absorbing species in solution: A=log10(I0/I)=εc L, where A is the measured absorbance, I0 is the intensity of the incident light at a given wavelength, I is the transmitted intensity, L the path length through the sample, and c the concentration of the absorbing species. For each species and wavelength, ε is a constant known as the molar absorptivity or extinction coefficient. This constant is a fundamental molecular property in a given solvent, at a particular temperature and pressure, and has units of 1/M*cm or often AU/M*cm.
The Beer-Lambert Law predicts a linear relationship between absorption and concentration and is useful for characterizing many compounds but does not hold as a universal relationship. A second order polynomial relationship between absorption and concentration is sometimes encountered for very large, complex molecules or simpler compounds at relatively high concentration. The Beer-Lambert law has implicit assumptions that must be met experimentally for it to apply. For instance, the chemical makeup and physical environment of the sample can alter its extinction coefficient. The chemical and physical conditions of a test sample therefore must match reference measurements for conclusions to be valid. The Beer-Lambert law also only applies to pure solutions and unencumbered absorbance. In the real world, scattering from particles and non-specific absorption contribute to measured values.
An apparatus for measuring the purity of fluids is known from the disclosure of U.S. Pat. No. 8,102,518 to Haught et al. Devices used for measuring fluid purity in general, and for identifying and quantifying the amount of impurities in particular, commonly use light as a probing mechanism. Such devices are generally referred to as photometers. A specific type of photometer is the spectrophotometer, which permits adjustment of the light frequency (i.e., wavelength), for making measurements at multiple frequencies. An optical sample cell contains a portion of fluid being analyzed at any given moment.
Electromagnetic energy that is used to irradiate the aqueous stream may either be reflected by material in the aqueous stream, transmitted through the aqueous stream and its material load, or absorbed by the aqueous stream material. In the instance where the electromagnetic energy is absorbed by the aqueous stream material, the aqueous stream material may also fluoresce. In devices used to measure purity, one of three basic measurement methodologies following from these potential interactions of the electromagnetic energy with the aqueous stream is generally employed. These methodologies measure the parameters absorption, reflectance, and fluorescence of the aqueous stream in the optical sample cell. In accomplishing the various methodologies, an electromagnetic energy detector has been disposed with respect to an electromagnetic energy transmitter so that the detector is optimally positioned to be responsive to the associated parameter.
It is desirable, then, for an apparent real time view of the TOC in an aqueous sample to be obtained from observations available in intervals of seconds or minutes and not the hours or days traditionally obtained from laboratory tools such those recommended in the SUVA specification.